The final stage in the drilling for hydrocarbons and the preparation for production is to lower a casing within the well bore to provide integrity to the subterranean formation. Completion of the oil or gas well is achieved by lowering a perforating gun to the proper location to perforate the casing and allow a hydrocarbon flow from the earth formation into the well bore. There are many well known perforating guns adapted to form holes through the walls of a casing. Such perforating apparatus is generally equipped with high explosive shaped charges which are effective to blast perforations through the casing. After the casing perforation has been completed, the perforating apparatus is either withdrawn or dropped into the well so that production tubing equipment can be used for extracting the hydrocarbon minerals from the cased well bore.
Casing perforating guns are highly developed to improve the efficiency of the perforating operation and to optimize the overall reliability and thus reduce misfiring of the gun. Perforating guns are generally operated in conjunction with firing apparatus fixed hereto to provide safety to personnel. The perforating gun is constructed so that it is triggered only on the successful firing of the firing apparatus. For safety reasons, the gun itself is often first lowered into the well bore to the proper location, and then the firing apparatus is lowered and joined to the gun. The perforating gun and firing apparatus then forms a unit which can be set off to blast perforations through he steel well casing. Alternatively, the firing apparatus and the perforating gun are attached together at the surface and conveyed either by a tubing string or wireline to the proper location in the cased well bore. Such an arrangement is shown in U.S. Pat. Nos. 4,484,639 and 4,770,246 assigned to Dresser Industries, Inc.
In the event of a failure of the firing apparatus or the perforating gun, a substantial amount of time and cost is involved in withdrawing the perforating equipment from the well, complete repair or replacement thereof, and the lower the apparatus back into the proper location of the casing. In certain instances, this can only be accomplished by drilling out various components, such as packers, to retrieve the perforating equipment. It can be appreciated that in drilling and preparing a well for production, the hourly cost may be in the order of $5,000, and thus the malfunction of perforating equipment can have a substantial impact on the overall cost of the operation. In addition, a misfiring or malfunction of the perforating apparatus often damages the equipment to the extent that it is not reusable.
As noted above, the casing perforating apparatus comprises a firing system and a perforating gun, the combination of which is effective to be triggered by an electrical current, fluid pressure or mechanical stimulus to blast holes in the casing. Firing apparatus is generally of rather complex construction, as noted in U.S. Pat. No. 4,484,639, which discloses a detachable firing apparatus and perforating gun. Electrical firing assemblies are generally responsive to an electrical current for setting off the perforating gun, while mechanical firing assemblies are set off by dropping a bar down a tubing string to which the perforating apparatus is connected. Fluid actuated firing assemblies are activated by pressurizing the tubing string or the annulus with a hydraulic fluid or gas.
Because of the ramifications of a failure of firing systems, attempts have been made to improve the reliability by providing dual-type firing systems. In the dual-type firing assemblies, the firing apparatus is duplicated so that if one part should fail, the other can be employed to trigger the perforating gun, without an intermediate tubing string retrieval and repair of the faulty firing mechanism. However, the provision of the dual-type firing system has not only rendered the apparatus more complex and costly, but often the misfiring of one firing assembly renders the other inoperative, generally due to a low-order internal explosion which failed to go high order. U.S. Pat. Nos. 4,632,034 and 4,678,044 each disclose redundant firing apparatus such that if one unit fails, the other can be activated to detonate the perforating gun. However, in the noted duplicated firing assemblies, one must be situated above the perforating gun and the other below, thus necessitating distinct assemblies and additional time, labor, and safety concerns to complete assembly of the unit at the well site.
U.S. Pat. No. 4,610,312 also discloses a redundant firing system which is constructed such that if a primary hydraulic firing head misfires, a secondary mechanical firing head can be activated to set off the perforating gun. However, due to the construction of such a firing head, and especially the arrangement of the detonating cord, it is probable that a low order misfiring of the primary firing equipment could render the secondary firing equipment ineffective to detonate the perforating gun. In addition, the redundant firing systems are not fluid isolated such that any pressurized fluid utilized in firing the hydraulic firing head, if leaked through any of the seal members, can wet the detonating apparatus such that one or both firing heads cannot be activated to detonate the perforating gun. The firing mechanism noted in the patent also requires a differential downhole pressure to activate the hydraulic firing head, which type of activation is susceptible to premature firing, as is well known in the art. Yet another disadvantage of the redundant firing system is that it cannot be retrieved, either by itself or with the perforating gun, from the cased well bore.
From the foregoing, it can be seen that a need exists for an improved dual-type firing system in which both firing assemblies are independent, but yet are housed in a single unit and thus connectable to one end of a perforating gun. A further need exists for a dual-type firing system in which any combination of mechanical and hydraulic firing assemblies can be employed, and in which a misfiring of the hydraulic firing assembly does not allow fluid to affect the firing capability of the other firing assembly. Another need exists for a dual firing system which overcomes the disadvantages of well known firing systems in that there are relatively few moving parts, and conventional hydraulic firing heads can be used as a unit within the firing assembly, thereby simplifying assembly thereof. Another need exists for an assembly in which the firing assembly is releasable from the perforating apparatus while down hole.